1. Field of the Invention
The present invention relates to a method of manufacturing a silicon solar cell, and, more particularly to a method of electrically insulating electrodes of the solar cell.
2. Description of the Related Art
In a silicon solar cell, light is converted into electricity by separating and collecting carriers (electrons and holes) with the aid of diffusion potential of a pn junction of the solar cell. Carriers are generated when light is irradiated to the pn junction.
Silicon solar cells can be of two types: front-surface-junction solar cells and back-surface-junction solar cell. In a front-surface-junction solar cell, as shown in FIG. 3, electrodes are formed on a front surface (light receiving surface) and a back surface. This solar cell has an n-type diffusion layer 2 on a front surface of a p-type silicon substrate 1, and a pn junction 3 in the boundary of the p-type silicon substrate 1 and the n-type diffusion layer 2. A first electrode 5 makes an electric contact with the n-type diffusion layer 2 and a second electrode 6 makes an electric contact with the p-type silicon substrate 1. An anti-reflection coating 4 is formed on the light receiving surface of the solar cell.
In a back-surface-junction solar cell, as shown in FIG. 4, electrodes are formed only on the surface opposite to the light receiving surface. This solar cell has the n-type diffusion layer 2 on the back surface and the edges of the p-type silicon substrate 1. The pn junction 3 is formed in the boundary of the p-type silicon substrate 1 and the n-type diffusion layer 2. A first electrode 5 and a second electrode 6 are formed on the back surface. The first electrode 5 makes an electric contact with the n-type diffusion layer 2 and the second electrode 6 makes an electric contact with the p-type silicon substrate 1. An anti-reflection coating 4 is formed on the light receiving surface.
The solar cells can be formed by various methods. Diffusion layer formation method, for example, is commonly used. Gas is used as a diffusion source in the diffusion layer formation method which makes this method cost effective and suitable for mass production. In the diffusion layer formation method, although a diffusion layer can be advantageously formed on the entire surface of the substrate, a short circuit is disadvantageously formed between the first electrode and the second electrode via the diffusion layer. FIG. 5 is a schematic of the short circuit.
Parallel resistance of the solar cell decreases due to formation of the short circuit, thereby reducing Fill Factor (FF) that directly contributes to conversion efficiency of the solar cell. Therefore, as described in Japanese Patent Laid-Open Publication No. H5-326990 (see Page 3, FIG. 1), a part of the diffusion layer needs to be removed to electrically insulate the first electrode and the second electrode from each other. In a method employed for a front-surface-junction crystalline silicon solar cell, silicon wafers are stacked and the diffusion layer on the edges is removed by means of plasma.
Thin silicon wafers are generally used to reduce cost. However, thin wafers are fragile and they can break easily. Moreover, when a structure is employed in which a plurality of silicon wafers are stacked, each silicon wafer needs to be handled separately.
Solar cells can be formed using etching methods (mask etching method). The etching methods include wet etching and dry etching. Wet etching includes etching using an acid or an alkali. Dry etching includes etching using Reactive Ion Etching (RIE). However, the etching methods include a lot of steps such as application of etching resist, drying, removal of etching resist, washing, etc. so that the etching methods are costlier.